Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems (TLS). Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400 μs and lifetime-limited coherence times approaching one millisecond in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of CT2=1.1×105. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.
@article{arxiv.2602.22117,
title = {Loss Mechanisms in High-coherence Multimode Mechanical Resonators Coupled to Superconducting Circuits},
author = {Raquel Garcia Belles and Alexander Anferov and Lukas F. Deeg and Loris Colicchio and Arianne Brooks and Tom Schatteburg and Maxwell Drimmer and Ines C. Rodrigues and Rodrigo Benevides and Marco Liffredo and Jyotish Patidar and Oleksandr Pshyk and Matteo Fadel and Luis Guillermo Villanueva and Sebastian Siol and Gerhard Kirchmair and Yiwen Chu},
journal= {arXiv preprint arXiv:2602.22117},
year = {2026}
}